The purpose of Bourbon Sass Mixins is to provide a comprehensive library of Sass
mixins that are designed to be as vanilla as possible, meaning they should not
deter you from using the original CSS syntax.
The mixins contain vendor specific prefixes for all CSS3 properties for support
amongst modern browsers. The prefixes also ensure graceful degradation for older
browsers that support only CSS3 prefixed properties. Bourbon uses SCSS syntax.

Rails 3.1.x

Bourbon needs the sass files to be imported in a specific order to function properly. Therefore, you will need to disable the require_tree sprocket directive.
Delete the following sprocket directive in application.css.scss:

Rails 3.0.x and below

For Rails < 3.1 you must run the installation rake task. Please note, you should run this task every time a new version of Bourbon is released.
This will copy the Sass files into your project's public/stylesheets/sass directory.

rake bourbon:install

Import the mixins at the beginning of your stylesheet

@import 'bourbon/bourbon';

Optional: If you use a non-standard location for your sass files, you can specify the path to your sass files as an argument to the install task

Help Out

Currently the project is a work in progress. Feel free to help out.
A general rule when considering filing a pull request for a new mixin: Do the following browsers only support the CSS3 property using vendor specific prefixes?
If the answer is yes, there is a high chance the mixin will be accepted via a pull request.